1. Field of the Invention
The present invention relates to a recording/reproducing separated type magnetic head having a magneto-resistive sensor mounted thereon, the sensor utilizing the giant magneto-resistive (GMR) effect or the tuning magneto-resistive (TMR) effect.
2. Description of the Prior Art
FIG. 10 shows a basic constitution of a magneto-resistive sensor of an existent spin valve structure disclosed in Japanese Patent Laid-Open Hei 3-125311. In a spin valve structure (magneto-resistive effect element) 2, a free layer 101 having soft magnetic properties and a pinned layer 103 fixed for the magnetizing direction are stacked by way of a non-magnetic conductive layer 102 on a seed layer 100, an anti-ferromagnetic layer 104 is stacked on the pinned layer 103, and a cap layer 105 is disposed on the anti-ferromagnetic layer 104. The anti-ferromagnetic layer 104 serves to fix the magnetizing direction of the pinned layer 103. Further, the stack 2 is formed to have a predetermined width (track width) and stacks of magnetic domain control layers (hard bias layer) 107 for controlling the magnetizing direction of the free layer 101 and electrode layers 110 for supplying a sense current are disposed on opposite sides of the stack above the seed layer 106. The stacks are put between a lower shield layer 112-b and an upper shield layer 112-a by way of a lower insulating layer 111-b and an upper insulating layer 111-a. 
The operation will be described briefly. The GMR effect is a phenomenon that resistance changes in accordance with the difference of angle between the magnetizing directions of the free layer 101 and the pinned layer 103. The resistance is lowest at an angle of zero where the magnetizing directions are identical, whereas the resistance is highest at an angle of 180° for anti-parallel state. Magnetization for the pinned layer 103 is fixed in the direction vertical to the track direction such that the angle is 90° when the external magnetic field is not applied.
The magnetizing direction of the free layer 101 is directed to the track due to the magnetic field from the hard bias layer 107 (longitudinal bias magnetic field) and the easy axis of the film itself. The difference of the angle in the magnetizing direction changes in accordance with the positive or negative directions of the external magnetic field, by which the resistance changes, and the sense current allows the change of the voltage across both ends of the magneto-resistive effect (GMR) element which operates as a magneto-resistive sensor.
Along with increase in the density and narrowing of the track width, the output from the structure described above lowers abruptly. This is because a dead region in which the sensitivity to signal magnetic fields is lowered is present at the end of the track width of the magneto-resistive element (sensing portion) 2 of a magneto-resistive sensor due to the intense longitudinal bias magnetic field generated from the hard bias layer 107. Further, since the gap Gs between the upper and lower shield layers changes scarcely while the track width has been narrowed recently, the longitudinal bias magnetic field remains even in the central portion of the track width of the sensing portion 2 as the case may be, making it difficult to improve the sensitivity.
FIG. 11 shows a normalized magnetic field distribution in the sensing portion for the track width Twr of 200, 150 and 100 nm at Gs of 60 nm in the sensing portion 2, and for Br·Th of 200 Gum, Br being residual magnetization of the hard bias layer 107 and Th being thickness of the hard bias layer 107. The ratio of the magnetic field at the center relative to the magnetic field at the end in the sensing portion 2 is 0.1 for the track width of 200 nm but the ratio increases as the track width is narrowed. The value increases to 0.3 for the width of 100 nm. This means that the sensitivity is reduced to about ⅓. Considering that the track width is narrowed to ½, the sensitivity is decreased to ⅙. To solve the lowering of the output, the longitudinal bias magnetic field is decreased by decreasing the residual magnetization or the thickness of the hard bias layer 107. While this can improve the output, the longitudinal bias magnetic field lowers. As a result, the magnetization at the track end is less directed to the track because of demagnetization. In the case where the magnetization of the free layer 101 is rotated by an external magnetic field, magnetization cannot be rotated smoothly at the track end if the longitudinal bias magnetic field is low. As a result, magnetization shows rotational operation with hysteresis to generate magnetic noises. As the case may be, asymmetricity with respect to the positive and negative output of the read waveform may also be increased. Along with increase in the output, the frequency for the occurrence of heads having noises or waveform asymmetry is increased and it is actually impossible to increase the output while lowering the longitudinal bias magnetic field.
To avoid lowering of the output with regard to the narrowing of the track, necessary resistance change can be obtained for the narrowing of track if the angle of the magnetization rotation to the external magnetic field can be ensured without increasing the longitudinal bias magnetic field from the hard bias layer at least in the sensing portion (magneto-resistive effect element). To prevent increase of the longitudinal bias magnetic field in the sensing portion, it is necessary to reduce the thickness of the hard bias layer in proportion with the track width. However, reduction for the thickness of the hard bias layer brings about lowering of the longitudinal bias magnetic field at both ends of the track. Accordingly, fixing for the magnetization at the track end becomes insufficient. This generates side reading. When fixing is further weakened, reproduction noises and asymmetrical fluctuation of waveforms increase. When side reading occurs, the effectively wide track width is required and an optically narrower track width is necessary for obtaining a required operation width to increase the burden on the process technology.
The problem includes reproduction noises and asymmetrical fluctuation of waveforms. The frequency for the occurrence of them is also determined by the longitudinal bias magnetic field at the end. When the thickness of the hard bias layer is reduced with an aim of improving the output, it naturally lowers the longitudinal bias magnetic field on the end and abruptly increases the reproduction noises and the asymmetry fluctuation of the waveforms. That is, there is a conflicting trade-off relationship at present between improvement in the output and noises, waveform asymmetricity or magnetic track width. If it is intended to increase the output, noises increases, the waveform asymmetry is increased and the magnetic track width is enlarged, making it actually difficult to improve the output.
As has been described above, the subject in the narrowing of the track width is to get out of the trade-off relation described above. However, the subject cannot be attained in the existent hard bias structure. In view of the above, this is attributable to that decrease of the longitudinal central bias magnetic field or decrease of the dead region for improving the output would lower the bias magnetic field at the end simultaneously and inevitably cause noises and waveform asymmetricity.